Usage notes

The term planet'' originally meant any star which wandered across the sky, and generally included comets and the Sun and Moon. With the Copernican revolution, the Earth was recognized as a planet, and the Sun was seen to be fundamentally different. The Galileian satellites of Jupiter were at first called planets (satellite planets), but later reclassified along with the Moon. The first asteroids were also thought to be planets, but were reclassified when it was realized that there were a great many of them, crossing each other's orbits, in a zone where only a single planet had been expected. Likewise, Pluto was found where an outer planet had been expected, but doubts were raised when it turned out to cross Neptune's orbit and to be much smaller than the expectation required. When Eris, an outer body more massive than Pluto, was discovered, the International Astronomical Union (IAU) officially defined the word ''planet as above. However, a significant minority have refused to accept the IAU definition. Many simply continue with the nine planets that had been recognized prior to the discovery of Eris. Others are of the opinion that orbital parameters should be irrelevant, and that any equilibrium (?spherical) body in orbit around a star is a planet; there are likely several hundred such bodies in the Solar system. Still others argue that orbiting a star should also be irrelevant, thus re-accepting the Galileian satellites (as well as a dozen other moons) as planets. Note that the 2006 IAU definition defines a planet in respect to the Sun, and is thus technically inapplicable to exoplanets.

Usage notes

The term "asteroid" has never been precisely defined. It was coined for objects which looked like stars in a telescope but moved like planets. These were known from the asteroid belt between Mars and Jupiter, and were later found co-orbiting with Jupiter (Trojan asteroids) and within the orbit of Mars. They were naturally distinguished from comets, which did not look at all starlike. Starting in the 1970s, small non-cometary bodies were found outside the orbit of Jupiter, and usage became divided as to whether to call these "asteroids" as well. Some astronomers restrict the term "asteroid" to rocky or rocky-icy bodies with orbits up to Jupiter. They may retain the term planetoid for all small bodies, and thus tend to use it for icy or rocky-icy bodies beyond Jupiter, or may use dedicated words such as centaurs, Kuiper belt objects, transneptunian objects, etc. for the latter. Other astronomers use "asteroid" for all non-cometary bodies smaller than a planet, even large ones such as Sedna and (occasionally) Pluto. However, the distinction between asteroid and comet is an artificial one; many outer "asteroids" would become comets if they ventured nearer the Sun. The official terminology since 2006 has been small Solar System body for any body that orbits the Sun directly and whose shape is not dominated by gravity.